1. Field of the Invention
The present invention generally relates to optical lithography useful for the manufacture of semiconductor integrated circuits and, more particularly, to economical and flexible diffraction techniques for annular illumination to achieve resolution enhancement in semiconductor manufacture.
2. Background Description
Optical lithography is widely used in the manufacture of semiconductor integrated circuits (ICs). An image of a pattern to be formed on a semiconductor substrate is projected at reduced scale on to the surface of the substrate which has been coated with a suitable photoresist. The optical system is typically incorporated in a step and repeat projector so as to pattern an entire semiconductor wafer in a series of indexed steps. Exposure and development of the photoresist is followed by the addition or removal of materials as, for example, by deposition or etching. In order to fabricate ICs of increased complexity while reducing feature size, it is important to improve the uniformity of the illumination and depth of focus of the image on the substrate.
Incorporation of alternate pupil fills requires major redesign of the illuminator system and results in limitations on pupil fill choices. These limitations include varying degradation of the illumination uniformity, modified distortion characteristics, and intensity loss with pupil fill choices. Typical solutions use filters that block most of the light from the illuminator or a rotating prism that provides one fixed type of off axis illumination.
Y. Oh et al. in "A New Mask Technique for Optical Lithography--Dummy Diffraction Mask", SPIE, Vol. 1927, Optical/Laser Microlithography VI, 1993, H. Kang et al., in "A New Method of Tilted Illumination Using Grating Mask; ATOM (Advanced Tilted Illumination on Mask)", SPIE, Vol. 1927, Optical/Laser Microlithography VI, 1993, and R. Pforr et al. in "New Resolution Enhancing Mask for Projection Lithography Based on In-situ Off Axis Illumination", SPIE, Vol. 1927, Optical/Laser Microlithography VI, 1993, have described techniques which minimized these limitations for dipole and quadrupole illumination through the use of diffraction grating; however, these papers do not describe diffraction techniques for annular illumination configurations.
R. Hollman in U.S. Pat. No. 5,359,388 describes a microlithographic projection system which uses a diffraction grating to eliminate the zeroth order for annular illumination; however, Hollman uses a kaleidoscope structure to replicate and overlap the diffracted orders which results in a complicated optical design.